I have found the purchase of one of the Brother P-Touch label makers, which come up on sale at the big discount stationery stores from time to time, to be a generally extremely useful item to have around the house. You can then get plastic label stuff in various sizes and colors, including white text on clear background or black text on white background or text on color backgrounds, to suit different purposes.
Not only could you label bottles in the darkroom, but you could label many other things around the house to make them easily identifiable, like distinguishing the many AC adapter wall warts that accompany so many products these days as for the device it is intended , or the number of hours charge time for the intended item.